The reaction of thermal decomposition of Ag4[Fe(CN)6] has been established on the basis of thermal and X-ray analyses of the solid decomposition products and chemical analysis of gaseous products. It has been found out that on the basis of the loss in mass of silver hexacyanoferrate(II) at 400-degrees-C it is possible to determine Fe(CN)6- ions in the presence of Cl-, Br- or I-, whose silver salts do not decompose at this temperature.